Two mutants were evaluated in seronegative chimpanzees. Both mutants retained some virulence for the upper respiratory tract suggesting that they were not sufficiently attenuated for use in susceptible infants and young children. The phenomenon of immunosuppression was studied in greater detail by administering pooled cotton rat RS virus antisera intraperitoneally (IP) to seronegative animals which were then immunized IM with live wild-type virus. Those animals which were given antisera were unable to respond to the IM vaccine, whereas the animals which received pooled control sera (free of RS virus antibody) prior to vaccination were successfully immunized. This demonstrated that serum antibody or a concomitant serum immune factor, could produce immunosuppression. The immunosuppressive effect seen in IM was not seen during IN vaccination, and thus topically applied RS virus vaccines may confer effective immunity independent of antibody in serum at the time of vaccination.